As a midsole structure for a shoe, for example, Japanese Patent Application Publication No. 2012-515621 discloses a midsole structure that comprises a shell having a central concavity formed inside an outer circumferential edge portion and an insert to be inserted into the central concavity of the shell (see paras. [0021], [0024], [0036] and FIGS. 2 to 6). Also, Japanese Patent Application Publication No. 2013-529535 discloses a midsole structure that comprises a main midsole having a slope portion, an upper midsole disposed on an upper side of the main midsole, and a lower midsole disposed on a lower side of the main midsole (see paras. [0025], [0031], [0032] and FIGS. 2 and 3).
With regard to the above midsole structure described in JP 2012-515621, in the structure of FIG. 5, the shell has a hardness greater than a hardness of the insert, and in the structure of FIG. 6, the insert has a hardness greater than a hardness of the shell. In either case, a boundary layer or interface between the shell and the insert that have a hardness difference is exposed to a top surface of the midsole. Similarly, with regard to the above midsole structure described in JP 2013-529535, as shown in a bottom drawing of FIG. 3 (or a cross sectional view of FIG. 2 taken along line C-C), a boundary layer between the main midsole and the upper midsole that have a hardness difference is exposed to a top surface of the midsole.
Therefore, according to the above-mentioned prior-art structure, in wearing a shoe as well as in impacting the ground, a foot sole of a shoe wearer contacts the boundary layer or interface between the two midsole layers having a hardness difference. Thereby, he/she may feel discomfort to the foot sole thus losing ride feeling during running.